Why does a three year old's knee hurt all the sudden?!

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Have her see a pediatric specialist in joint disorders just to rule out anything like juvenile arthritis (yes, there is such a thing.) And it's probably not "growing pains", which are concentrated in the muscles and not the joints according to the source I posted below.
If nothing comes up (Which I doubt) then she might just be a very very active child that is continually hurting herself- but if it's just the knees, which are two very large joints, then I'd be concerned.

I had juvenile rheumatoid arthritis.
http://www.arthritis.org/ja-disease-type…
http://kidshealth.org/parent/general/ach…

Just means she's growing, called growing pains (:

Growing pains or sprained knee

due to increase in pain and unhealed wounds and under developed bones